1. Field of the Invention
The invention relates to nuclear reactors and more particularly to improved control rod drive shaft guides.
2. General Background
In reactors such as those made by Westinghouse, the control rod drive shaft guide consists of a monolithic ring centered by the through hole in the top card of the control rod guide structure. The guide is held in place by four bayonet-like devices which are attached to the card. Conical heads on the bayonet-like devices are pushed aside as the ring is installed and snapped over the edge of the ring to hold it down. In this design, the bayonet-like devices are highly stressed during operation due to differential thermal expansion and are prone to damage during the installation and removal of the ring. As a result of these various stresses, many of the bayonet-like devices in operating nuclear reactors become bent, cracked or broken.
Several patents in the known art present guide structures.
U.S. Pat. No. 4,326,921 entitled "Control Rod Guide Thimble for Nuclear Reactor Fuel Assemblies" teaches improved control rod guide thimbles which include insets which project inwardly from the guide thimble to reduce control rod end vibrations during operation and thus reduce thimble wear.
U.S. Pat. No. 4,030,975 entitled "Fuel Assembly for a Nuclear Reactor" teaches a threaded connection formed between the upper and/or lower alignment posts and the guide tubes which clamps the fuel assembly end plate therebetween.
U.S. Pat. No. 4,313,796 entitled "Guide Post Modification for Nuclear Fuel Assembly" teaches an improved guide tube post which decreases the flow induced vibration of a cylindrical control rod suspended therethrough. The inner wall of the post, near where it exits into a diffuser region, is provided with a polygonal cross section.
U.S. Pats. No. 4,208,248 entitled "Fuel Assembly With a Removable End Fitting" and 4,321,111 entitled "Industrial Technique" teach a fuel assembly having a removable end fitting.
The above patents do not provide for elimination of highly stressed materials in the retention of the drive shaft guide and immunity to damage of the guide during installation or removal.